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Chapter 21 Temperature, Heat and Expansion

Chapter 21 Temperature, Heat and Expansion. Conceptual Physics Hewitt, 1999 Bloom High School. 21.1 Temperature. Temperature- a measure of how hot or cold something is Demonstrated by the expansion or contraction of a liquid Red thermometer- colored alcohol Silver thermometer- mercury

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Chapter 21 Temperature, Heat and Expansion

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  1. Chapter 21Temperature, Heat and Expansion Conceptual Physics Hewitt, 1999 Bloom High School

  2. 21.1 Temperature • Temperature- a measure of how hot or cold something is • Demonstrated by the expansion or contraction of a liquid • Red thermometer- colored alcohol • Silver thermometer- mercury • Celsius scale- 100 degrees difference between freezing and boiling of pure water • 0°C corresponds to the freezing point of pure water • 100°C corresponds to the boiling point of pure water • Fahrenheit scale- 180 degrees difference between freezing and boiling of pure water • 0°F corresponds to a mixture of ice, water, ammonium chloride and salt • 100°F corresponds to human body temperature

  3. Kelvin • Kelvin- 100 degrees difference between freezing and boiling of pure water • 273K corresponds to the freezing point of pure water • 373K corresponds to the boiling point of pure water • 0K (absolute zero) represents zero kinetic energy of a substance • Fahrenheit is an English unit • Celsius is not SI, but a derived unit • Kelvin is SI (metric) • We don’t use “degrees” with Kelvin

  4. Temperature Conversions °F = (1.8 x °C) +32 °C = (°F-32) x 0.555 Kelvin = °C + 273.2

  5. Temperature & Kinetic Energy • Temperature is related to the motion of molecules • Proportional to the average kinetic energy • Not a measure of the total KE of the molecules • Example: There is twice as much energy in 2L of 90°C H2O as 1L of 90°C H2O • Even though the temperature is the same, there is twice as much mass

  6. 21.2 Heat • Heat- the energy transfer that takes place because of the temperature difference between two objects • Energy always flows from high to low • Matter does not contain heat • Thermal energy- energy resulting from heat flow • Chapter 8 • Internal energy- term used in this chapter for the same thing • Thermal contact- when heat flows due to direct contact

  7. 21.3 Thermal Equilibrium • Thermal equilibrium- when objects in contact reach the same temperature • No heat flows from one to the other any more • A thermometer is read at thermal equilibrium • When the alcohol or mercury stops moving • The glass of the thermometer is too small to affect the temperature

  8. 21.4 Internal Energy • Internal energy- grand total of all of the energies in the substance • Translational energy (moving in a straight line) • Rotational energy of the molecule • Kinetic energy within the molecule • Potential energy due to forces within molecule • As a substance absorbs energy, the temperature increases • As a substance gives off energy, temperature decreases and one or more of the energies decrease

  9. 21.5 Measurement of Heat • Temperature- a measure of heat transfer, not heat content • Calorie (cal)- amount of heat required to increase 1g of H2O by 1°C • Metric, derived unit of heat energy • kilocalorie (kcal or Cal)- 1000 calories • Found on food wrappers • Joule (J)- 4.18J are required to increase 1g of H2O by 1°C • Standard SI unit of heat energy Hungry Man Classic Fried Chicken Dinner

  10. 21.6 Specific Heat Capacity • Specific heat capacity (or just specific heat)- the quantity of heat required to raise the temperature of 1g by 1°C • Ability to store internal energy • A measured value for each substance (see table)

  11. Using Specific Heat (chemistry review)

  12. 21.7 The High Specific Heat Capacity of H2O Absorbs more than would be expected Slow to gain heat Slow to lose heat Air near water areas are cooler than far inland because water resists a change in temperature

  13. “Cooler near the lake”

  14. 21.8 Thermal Expansion • As temperature increases, kinetic energy of the molecules increase and they “wiggle” further apart • Gases expand the most when heated • Liquids expand less than gases • Solids expand less than liquids

  15. Your Thermostat & You • Bimetallic strip- a strip made of two metals that expand at different rates • Brass & iron, typically • One side expands at a different rate than the other • Thermostat- practical application of bimetallic strip to turn on and off your furnace/AC • In the diagram, closing the circuit (touching) turns the furnace on

  16. Thermostat Cat

  17. 21.9 Expansion of Water • All liquids expand when heated • Ice cold (0°C) H2O does the opposite! • As H2O warms from 0°C to 4°C, it contracts • H2O is more dense at 4°C than 0°C, therefore sinks to the bottom of the container

  18. Solid versus Liquid H2O • In liquid H2O, some of the O’s are attracted to the H’s • H2O molecules are able to slide past each other with minimal interaction • In solid H2O, each O is attracted to two H’s • This causes large, fixed gaps, which decreases density • Ice floats because of this decreased density • Lakes/ponds freeze from the top down

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